1. Field of the Invention
The invention relates to the manufacture of electric lamps, and is directed more particularly to a method for recrystallizing incandescent lamp tungsten filaments so as to provide a filament 9 having improved sag characteristics and resistance to cold fractures.
2. Description of the Prior Art
A typical incandescent gas-filled lamp produced today is provided with a filament made from potassium-doped grade non-sag (NS) tungsten wire of either a single coil or coiled-coil design. To develop a sag-resistant metallurgical structure, the filament must be recrystallized prior to its regular operation in a lamp. In a typical process, the tungsten coil is first mounted. The mount assembly is then hermetically press-sealed inside a glass tube at one end (the press seal end) and left open at the other end (the exhaust tube end). This glass-mount assembly is commonly referred to as pressware. The pressware is attached to exhaust-flush-fill equipment wherein a number of operations are performed to clean the coil, introduce a getter, and add the fill gas. In the last operation of exhaust, the pressware is frozen, thereby condensing the fill gas and creating a vacuum. The exhaust tube is heated and the glass collapses from external pressure, sealing the lamp (referred to as "tipping off"). At some point after tip off, the tungsten coil is recrystallized in the lamp atmosphere. In high volume lamp manufacturing, it is common practice to recrystallize ("flash") the filaments in fully assembled and sealed lamps before shipping. Although recrystallized NS tungsten coils are inherently brittle and break easily by shock or impact, they usually survive inplant drop tests and shipping to a customer's location. However, occasionally, a high rate of coil breakages during the drop tests or shipping is encountered.
Another common defect of recrystallized NS tungsten coils is excessive sag or creep during lamp operation, which is caused by sliding of tungsten grains relative to one another, thus distorting the filament geometry and often resulting in a collapse of adjacent coil turns and premature failures. A prominent feature found in lamp coils that have failed, either in drop tests, shipping, or due to sag, is a predominance of intergranular fractures, indicating that poor grain boundary strength is the common cause of failures. In contrast, fully recrystallized lamp coils that perform well in drop tests, shipping, and sag, when fractured by stretching or impact, usually exhibit long coiled segments and predominantly transgranular fracture surfaces.
Tungsten has very low solubility for interstitials, such as oxygen and carbon. Once the solubility limits are exceeded, tungsten oxides and carbides precipitate out of the tungsten matrix and deposit on grain boundaries, resulting in intergranular brittleness at ambient temperatures and loss of creep resistance at high temperatures. Trace amounts of residual air and moisture in incandescent lamps, including tungsten halogen, adversely affect properties and performance of coiled tungsten filaments. Interactions between these residuals and a filament cause brittleness, loss of sag resistance, bulb blackening and premature lamp failures. Particularly damaging to tungsten filaments is an interaction with oxygen and/or water during "flashing". During flashing, a nonsag, large, interlocking grain structure is formed in a coiled filament. This process is characterized by a rapid and massive movement and growth of the primary tungsten grains. At this stage, the available oxygen reacts with tungsten grain boundaries and other surfaces, producing weak, noncoherent grain boundary phases and, hence, intergranular brittleness and partial or total loss of sag resistance. Conventionally, coil flashing in mass-produced incandescent and tungsten halogen lamps consists of resistance heating of coiled filaments to, or above, the recrystallization temperature in fully assembled and sealed bulbs containing a fill gas. Usually, a fill gas comprises a mixture of an inert gas such as argon, krypton or xenon, nitrogen, and a halogen compound (in tungsten halogen lamps). Solid/gaseous oxygen and water getters are also present. Depending on the lamp type, the pressure inside the bulb ranges between below 1 atmosphere to a few atmospheres. Such fill gas compositions, while not damaging to tungsten, are not effective in removal of impurities from filaments, bulb walls or other parts of the lamp. Although the getters perform this function, the gettered impurities remain in the vessel for the life of the lamp. Because of the sporadic amounts of air/moisture residuals in mass-produced lamps, even in the presence of getters, fully flashed tungsten filaments are often excessively brittle, as evidenced by the high rates of coil fractures during lamp drop tests or shipping, and exhibit high sag rates during life.